I'm not a diesel expert, but I do know that there are a lot more problems with diesel calibrations with respect to the torque developed. A lot of calibrations have some from of torque limiting built in to give the gearbox/powertrain more chance of surviving, and I suspect that the power chips available get most of their gains by overiding these limits.

Emissions is also quite a tricky area and increasing the power will almost certainly involve a different compromise. Probably not an issue for the MOT, but not environmentally friendly.